The analysis of stable oxygen isotopes in natural systems allows for the elucidation of plant physiological responses to environmental change from the scales of enzymes up through whole plants and ecosystems to the planet. Plants modify the distinct regional and seasonal isotopic signals in environmental water yielding unique tracers in atmospheric CO2, O2 and plant organic material. These signals allow for the partitioning of gross CO2 fluxes on a variety of temporal and spatial scales, the reconstruction of growth environment from seasons to centuries and the estimation of global productivity over millennia. The isotope ratio of leaf-water is central to all of these applications, and is controlled by three physical factors: (i) relative humidity, (ii) the isotope ratio of plant root water and (iii) the isotope ratio of atmospheric water vapor. Our lack of knowledge of the isotope ratio of atmospheric water vapor at any spatial or temporal scale leads to large errors in the interpretation of stable isotope ratios of CO2, O2, and plant organics. This error would be minimized by obtaining integrated estimates of the isotope ratio of atmospheric water vapor through time and space. The objectives of the proposed research are to develop and test models to explain the observed oxygen isotope ratios of leaf water and cellulose of the epiphytic CAM plant Tillandsia usneoides. It is hypothesized that the isotope ratio of leaf water and consequently leaf organic material is controlled by and reflects the isotope ratio of atmospheric water vapor. These predictions will be tested through a combination of laboratory experiments and field campaigns. Contemporary values of the isotope ratio of atmospheric water vapor for annual and monthly timescales will be determined at two isotopically distinct locations in Virginia and Florida; historical values in these regions will be reconstructed using herbarium specimens.
The broader impacts of this research represent a characterization of mean values for the oxygen isotope ratio of atmospheric water vapor on an unprecedented spatial and temporal scale. The realized final product of this leaf-level ecophysiological research would be a map of time-integrated atmospheric water vapor from Virginia, USA southwards through the tropics to Argentina. Such a product would benefit broad disciplines of scientific pursuits including ecosystem to global scale estimates of biological productivity, forensic applications, reconstruction of past climates and hydrological models. Additionally, this proposal will train a technician, a graduate student and several undergraduate students in the theory and applications of stable isotopes. These specialized analytical skills are highly marketable in both the public and private sectors.
